FISHERY BULLETIN: VOL. 69, NO. 3 



the age of skipjack tuna at various sizes. The 

 growth rate of a related species, bhick skipjack, 

 Euthynnus lineatus, between 17 and 75 mm SL 

 has been determined (Clemens, 1956). In ex- 

 periments conducted in shipboard aquaria Clem- 

 ens showed that black skipjack grew 36.8 mm in 

 295 hr, which would be about 9 cm in a month. 

 Houde and Richards (1969) reared larval little 

 tunny, E. alletteratus, under laboratory condi- 

 tions and reported similar growth rates. It was 

 recently found that larval skipjack tuna have 

 similar growth rates (personal communication, 

 William J. Richards, Supervisory Zoologist, 

 NMFS Tropical Atlantic Biological Laboratory, 

 Miami, Fla. 33149, August 18, 1970). If larval 

 skipjack tuna grow at the same rate (9 cm per 

 month) for the first month and 2 cm per month 

 for the next 11 months, then skipjack tuna 1 year 

 old would be 31 cm SL. 



Brock (1954) assumed that the modal sizes 

 of fish between 40 and 50 cm FL (fork length) 

 that appear in the Hawaiian skipjack tuna fish- 

 ery during the summer represented 1-year-old 

 fish. Forty to fifty centimeters for 1-year-old 

 fish seems too high. Modal lengths typical of 

 winter skipjack tuna in the Hawaiian fishery are 

 35, 50, and 70 cm FL (Rothschild, 1965). My 

 data indicate that the 35-cm modal group that 

 appears in the winter represents 1-year-old fish. 



SPAWNING 



In the Hawaiian area, juvenile skipjack tuna 

 smaller than 10 cm were found during 7 months 

 in 1963 and during 5 months in 1964 and 1965 

 (Figure 6). This suggests a protracted spawn- 

 ing season and nearly continuous recruitment 

 of juveniles. The catch of larvae indicates that 

 spawning begins in March, peaks in July, and 

 declines sharply in September and October 

 (Matsumoto, 1966). 



The Hawaiian ski|5jack tuna fishery peaks in 

 the summer when the bulk of the catch is com- 

 posed of "season-size" fish larger than 60 cm 

 (Rothschild, 1965). Larval and juvenile skip- 

 jack tuna also are most numerous during the 

 summer, which suggests that the large fish 

 spawn in Hawaiian waters. The presence of 



juveniles in the spring and fall, although in lesser 

 numbers, indicates that spawning also takes 

 place then. Rothschild (1965) has hypothesized 

 that at least one subpopulation spawns in Ha- 

 waiian waters. The protracted spawning sea- 

 son may indicate that more than one subpopu- 

 lation spawns here. 



In the South Pacific between lat 5° and 20° S, 

 the spawning season is more protracted than in 

 Hawaiian waters. Juveniles smaller than 10 cm 

 SL were taken in almost every month north of 

 lat 10° S. Between lat 10° and 20° S they were 

 taken in all months except March, April, June, 

 and July (Figure 8); the reason for their ab- 

 sence in these months may be inadequacy of 

 samiiling. The area south of lat 20° S was some- 

 what different. Although sampling was sparse, 

 the length-frequency distribution consistently 

 showed few small juveniles, which indicates little 

 or no spawning in this area. 



The length-frequency distribution of juvenile 

 skipjack tuna north of lat 10° S and between lat 

 10° and 20° S indicated a progression of modes 

 (Figure 8). However, a plot of the modal 

 lengths indicated no growth. The cause of this 

 may be a protracted spawning season and in- 

 adequacy of samples. Also juveniles appear to 

 migrate southward as they grow. Only small 

 numbers of juveniles largei' than 20 cm SL were 

 taken north of lat 20° S, and most of the ju- 

 veniles taken south of lat 20° S were larger than 

 20 cm SL. 



ACKNOWLEDGMENTS 



Thanks are due the research assistants at the 

 NMFS Hawaii Area Fishery Research Center 

 who examined the billfish stomach contents for 

 juvenile skipjack tuna and aided in summarizing 

 part of the data. The portion of the study in 

 the South Pacific would not have been possible 

 without the cooperation of the crews of the many 

 fishing vessels from Jajian and the Republic of 

 Korea. I thank William J. Richards, NMFS, 

 Miami, Fla.; Walter T. Pereyra. NMFS, Seattle, 

 Wash.; and Witold L. Klawe, Inter-American 

 Tropical Tuna Commission, La Jolla, Calif, for 

 reviewing the manuscript. 



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